In internal combustion engines, which can be operated with a lean air/fuel mixture (lambda>1), nitrogen oxide (NOx) storage catalytic converters are used in order to store the nitrogen oxide (NOx) emissions which are discharged by the engine during a first operating phase. This first operating phase of the NOx storage catalytic converter is also characterized as the storing phase. With increasing duration of the storing phase, the efficiency of the NOx storage catalytic converter falls off, which leads to an increase of the NOx emissions rearward of the NOx storage catalytic converter. The cause for this reduction of efficiency lies in the increase of the nitrogen oxide (NOx) fill level of the NOx storage catalytic converter. The NOx fill level can be monitored and the second operating phase of the NOx catalytic converter (discharge phase) can be initiated after a pregiven threshold value is exceeded. A nitrogen oxide (NOx) storing model can be used for determining the NOx fill level of the NOx storage catalytic converter.
During the second operating phase, a reducing agent is added to the exhaust gas of the internal combustion engine which reduces the stored nitrogen oxides to nitrogen and oxygen. Hydrocarbon (HC) and/or carbon monoxide (CO) can be used, for example, as a reducing agent. This reducing agent can be generated by a rich adjustment of the air/fuel mixture in the exhaust gas. Alternatively, urea can be added to the exhaust gas as a reducing agent. Here, ammonia from the urea is used for reducing the nitrogen oxide to oxygen and nitrogen. The ammonia can be obtained by hydrolysis from a urea solution.
Toward the end of the discharge phase, a large portion of the stored nitrogen oxide is reduced and less and less of the reducing agent comes together with nitrogen oxide, which it can reduce to oxygen and nitrogen. As a consequence, the portion of the reducing agent in the exhaust gas rearward of the NOx storage catalytic converter increases toward the end of the discharge phase and the portion of oxygen in the exhaust gas rearward of the NOx storage catalytic converter becomes less. From an analysis of the exhaust gas rearward of the NOx storage catalytic converter by suitable exhaust-gas sensors, the end of the discharge phase can be initiated when the largest portion of the nitrogen oxide has been discharged from the NOx storage catalytic converter.
In an NOx storing model, which is known from the state of the art, the NOx fill level of the NOx storage catalytic converter can be determined in dependence upon, inter alia, the NOx mass flow forward of the NOx storage catalytic converter, the NOx mass flow rearward of the NOx storage catalytic converter and the temperature of the NOx storage catalytic converter. From these variables, an efficiency of the NOx storage catalytic converter is determined. This efficiency is multiplied by the NOx mass flow forward of the NOx storage catalytic converter and is integrated to supply the actual NOx fill level. The second operating phase is initiated as soon as the NOx fill level exceeds the pregivable threshold value. For constant boundary conditions, the efficiency of the NOx storage catalytic converter decreases with increasing fill level.